Certain particulate hexavalent compounds cause lung cancer and respiratory lesions in an occupational setting. Hexavalent chromium enters the cells and is metabolically reduced to trivalent chromium. This process generates transient intermediate oxidation states of chromium and, possibly, reactive oxygen species (ROS). The resulting genetic and cellular damage inhibits cellular DNA replication and transcription, blocks the cell cycle, and causes apoptosis. The overall goals of this proposal are: (1) To investigate the cellular effects of soluble and particulate chromates in two normal human lung cell types that are likely to be targeted by chromium, the normal human lung fibroblasts and the small airway epithelial cells. The role of ROS and DNA-DNA interstrand crosslinks (DDC) in chromium-induced apoptosis, will be studied at the cellular level by using pharmacological antioxidants and Fanconi anemia cells that are deficient in the repair of DDC, respectively. (2) To investigate how DNA-bound chromium affects genomic replication. This will be modeled in vitro by replicating Cr-treated DNA by the human DNA replication complex, the synthesome. Thus, the proposed study will elucidate some of the cellular and molecular mechanisms of chromium-induced genotoxicity and apoptosis in the relevant, target human lung cells. From a public health perspective, this information will be important since it will contribute to our knowledge of the mechanisms of chromium toxicity and carcinogenesis in exposed humans.